A swirling flow is known as a fluid flow. FIG. 1 shows one example of a fluid flow in a state in which only a swirling flow exists. FIG. 1 shows a velocity vector at each of a plurality of node points (coordinate points) on one of swirling planes (planes normal to the swirling axis of the swirling flow) of the swirling flow. The direction and length of each arrow indicate the direction and magnitude of each velocity vector, respectively. As shown in FIG. 1, in the swirling flow, the fluid swirls around the swirling axis P0 as the center.
FIG. 2 shows one example of a fluid flow in a state in which a uniform flow is superimposed on the swirling flow. FIG. 2 shows a velocity vector at each of a plurality of node points (coordinate points) in a case where the uniform flow, which flows in the lateral direction from the left side to the right side of the figure, is superimposed on the swirling flow shown in FIG. 1. As shown in FIG. 2, the presence of the uniform flow besides the swirling flow results in failure to visually identify the presence of the swirling flow.
There has been a demand for a technique capable of adequately identifying the swirling flow by using a velocity vector at each node point (coordinate point). There has been a demand for a technique capable of, even under the presence of another flow, adequately identifying the swirling flow. There has been a demand for a technique capable of, even under the presence of another flow and a plurality of swirling flows, adequately identifying these swirling flows.
As a related art, D. Sujudi and R. Haimes disclosed a method for identification of swirling flow in “Identification of swirling flow in 3-D vector fields” (12th AIAA Comput Fluid Dyn conf 1995 Part2, (1995), p. 792-799). With this method, at first, an eigenvalue of a velocity gradient tensor are calculated. Next, if the eigenvalue is a complex number, a coordinate point on a swirling plane, where the velocity is 0 around the coordinate point, is searched for. The coordinate point where the velocity is 0 is defined as the position of a swirling axis. However, with this method, it is difficult to obtain the swirling axis under the presence of a uniform flow.
Moreover, R. C. Strawn, D. N. Kenwright, and J. Ahmad disclosed another method for identification of swirling flow in “Computer visualization of Vortex Wake System”, (AIAA (1999), vol. 7 (No. 4), p. 511-512). With this method, a vorticity ω is calculated. Then, a coordinate point indicating a local maximum of the vorticity ω is defined as the position of a swirling axis. Note that the vorticity ω represents (the twice of) an angular velocity in arbitrary pattern of flow which is not limited to a swirling flow. Thus, a flow having the vorticity ω not necessarily is a swirling flow. That is, having the vorticity ω is not a sufficient condition for being a swirling flow.
Furthermore, S. Kida and H. Miura disclosed still another method for identification of swirling flow in “Identification and analysis of vortical structure”, (E. J. Mech. B/Fluids, (1998), vol. 17 (No. 4), p 471-488). With this method, a swirling axis is obtained based on the Hessian of pressure, a vorticity and a velocity gradient tensor. However, not only velocity data but also pressure data is required, and the calculation is complicated. It is difficult to acquire pressure data at the necessary and sufficient number of coordinate points required for the analysis to obtain the swirling axis based on experimental data.